1. Field of the Invention
The invention relates to the field of casting mold materials. Particularly, the invention relates to a steel useful in connection with pressurized casting and corresponding methods.
2. Background of the Invention
In the description of the background of the present invention that follows reference is made to certain structures and methods, however, such references should not necessarily be construed as an admission that these structures and methods qualify as prior art under the applicable statutory provisions. Applicants reserve the right to demonstrate that any of the referenced subject matter does not constitute prior art with regard to the present invention.
In pressure casting and comparable methods, stresses on the mold are caused by the cyclic thermal shock due to the contact between molten metal and the mold steel, hydrostatic pressure due to the injection pressure, as well as mechanical and chemical abrasion of the mold surface due to the flow of molten metal. Mechanisms of mold damage are thermal fatigue, macrocracking and so-called wash out occurring as a consequence of erosion, corrosion and welding phenomena. The dominant mechanism of damage is partly dependant on the cast metal, the size and shape of the mold and the mold material. The most common cause of damage is hot cracking, which is the cause of about 85% of damage cases.
Hot cracking is reticular cracking on the mold surface caused by thermal fatigue. Unlike ordinary fatigue, thermal fatigue is not due to fluctuating external stresses, but the cyclic tension and distortion resulting in cracking is caused by temperature variations. On the basis of theoretical studies, it can be concluded that from the point of view of hot cracking resistance, the yield strength of the mold material should be high and as independent as possible of temperature and number of cycles, i.e. the material should be thermally stable.
In addition to hot cracking, wash out is another main mechanism leading to mold damage. Wash out refers to the removal of material from the mold surface due to the interaction between molten metal and the mold material. It has been established that corrosive, erosive and welding mechanisms are involved, and that it occurs mainly at sites where the mold material interacts strongly with the molten metal, as in the feed region and in cores. For wash out resistance, the hardness of the mold material should be high and the mold material should not easily form compounds with the molten metal.
Additional desirable material properties for pressure mold steels are as follows:
high yield strength
good ductility
good heat conductivity
good hot erosion resistance
small heat expansion coefficient
small size, even distribution and stable structure of precipitates
matrix stability
small solubility of mold material alloying elements in the metal subject to pressure molding
low level of impurities and good slag purity
homogeneous structure
Generally, it can be said that the properties of a mold steel are determined by the composition and the method of preparation, as well as the hot working and annealing.
The use of conventional maraging steels as a mold material is limited by the fact that the martensitic microstructure is not stable at temperatures above 480xc2x0 C. Above this temperature, the martensitic structure slowly begins to change into an austenitic structure. Austenite has different properties from those of martensite; the strength and thermal conductivity are lower, larger thermal expansion etc., and these deviating properties cause local tensions which accelerate the development of thermal cracks on the mold surface and thus shorten the service life of the mold.
The austenization temperature of Fexe2x80x94Ni, Fexe2x80x94Cr and Fexe2x80x94Nixe2x80x94Crxe2x80x94 based maraging steels is lowered particularly by nickel (about 10xc2x0 C. per weight-%) and chromium, however notably less by the latter than by the former. On the other hand, nickel and chromium particularly enhance the ductility of maraging steels. The austenization temperature of maraging steel can thus be raised by lowering the nickel content and/or by replacing part of the nickel with chromium. Simultaneously, care must be taken that the other properties of the steel remain on the appropriate level, by means of other alloying components.
A precipitate hardened mold steel of the maraging type, containing titanium, cobalt, chromium and nickel, has been invented having, in addition to high strength, good ductility, small thermal expansion coefficient and good thermal conductivity, a significantly better thermal stability than other maraging steels, and thus a better resistance to thermal cracking and wash out than conventional maraging steels.
A maraging-type mold steel according to this invention, containing titanium, molybdenum, cobalt, chromium and nickel, is prepared by a method that allows minimal impurity content of solid elements like carbon, phosphorus, sulphur, silicon, manganese and copper, and of gaseous elements like oxygen, nitrogen and hydrogen. Preferably, vacuum induction melting (VIM) is used, complemented by vacuum re-melting (VAR).
A maraging type mold steel according to the invention contains, in weight percent, no more than 0.03, preferably no more than 0.02% carbon; 9-18, preferably 10-14% nickel,; 1-5, preferably 1-3% chromium; 2-8, preferably 2-5% molybdenum; 5-15, preferably 10-12% cobalt; and 0.1-1.5, preferably 0.2-0.7% titanium. Preferably, the ratio Ni/Ti is in the range 15-20.
Preferably, a steel according to the invention additionally contains, in weight percent, no more than 1.0, preferably no more than 0.2% aluminum; silicon and manganese together no more than 0.20%, preferably no more than 0.15%; sulphur no more than 0.010%, preferably no more than 0.003%; phosphorus no more than 0.010, preferably no more than 0.005; the residue being iron and possible impurities.